Mechanical face seals are widely used to provide a seal between a stationary housing, often containing a lubricant such as oil, and a rotating shaft which passes through the housing. Typically, such seals include a stationary sealing ring about the shaft but fixed to the housing. Another sealing ring is mounted on and rotates with the rotating shaft. The seal face of the rotating sealing ring or the stationary sealing ring may include grooves of some type. These grooves form a layer of air between the sealing ring faces during operation. In these “hydrodynamic” seals, the sealing faces of both sealing rings are biased together by mechanical means that may include a spring.
In a magnetic face seal, in contrast, the stationary sealing ring is magnetic and attracts the rotating sealing ring. Thus, mechanical means to bias together the sealing faces can be eliminated. There are no hydrodynamic grooves in a magnetic face seal. Instead, the rotating sealing ring includes a carbon element and the seal face of the magnetic sealing ring rubs against the carbon element.
While magnetic face seals provide the advantage of utilizing magnetic forces to bias the sealing ring faces together, magnetic seals suffer from a number of disadvantages. For example, during operation the sealing faces contact each other, causing friction and generating heat. Consequently, the rings wear. The carbon element reduces but does not eliminate friction. Also, the carbon element can blister, causing further friction and heating. Thus, the effectiveness of the seal is decreased. Moreover, in the example where oil is the liquid to be sealed, the heat generated by the face seal can degrade the oil and result in a buildup of grime or “coking”. In each of these cases, the effectiveness of the seal is reduced, and in the worst case, catastrophic failure of the magnetic seal may occur.